Why does the alien hatch from the egg wearing a full jumpsuit?

umm yes.

I was wondering the same thing at first, but there was one quick bit of dialog that explains it, and it's very easy to miss. Someone mentions that the alien sleeps in the egg. I guess the alien creates a new egg every night to sleep in.

Stupid? Yes. But at least they gave an explanation for why it had clothes on coming out of the egg.

Plus, it was established that the alien was the co-pilot of the ship - thus it was someone who was already "born". So, unless there was a baby alien being hatched (a second alien), it would make sense that the egg is a disposable sleeping chamber.
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Paul R.

http://spookcentral.cjb.net

I dunno.

She sleeps in the egg thing at night. They wake up in the tomb and find the egg shells and speculate that it coccoons at night.